1. Field of the Invention
The current invention relates to communication networks capable of transmitting electrical power along with data, and more particularly, to systems and methods for the provision of electrical power in Power-over-Ethernet (PoE) systems.
2. Description of the Related Art
A Power-over-Ethernet system is an Ethernet network capable of transmitting both data and electrical power over twisted pair wires, such as category 5 cables. Ethernet is currently defined by the IEEE 802.3 standard, and PoE is currently defined by the IEEE 802.3af standard, both of which are incorporated herein by reference. Using PoE allows for the convenient delivery of electrical power to Ethernet client devices, such as Internet telephones or cameras, that may otherwise require more cumbersome powering arrangements in order to operate. PoE allows for the delivery of electrical power using the same cables that deliver Ethernet data.
FIG. 1 shows a simplified block diagram of conventional PoE system 100. PoE system 100 comprises power sourcing equipment (PSE) device 101, powered device (PD) 102, and unshielded twisted pair (UTP) category 5 cable 108. PSE device 101 comprises power supplies 105 and 106, PSE module 104, PHY module 103, and RJ45 interface 107. PD device 102 comprises PHY module 110, PD module 109, bridge circuit 111, signature circuit 112, and RJ45 interface 113. Cable 108 connects PSE device 101 to PD device 102. PoE system 100 is shown in FIG. 1 adapted to employ the PoE optional connection method called Alternative B, in which power is carried over the so-called spare pairs (wire pairs 4/5 and 7/8). The so-called spare pairs are used as spares in 10 MBit and 100 MBit Ethernet systems, and are used for data transmission in Gigabit (1000 MBit) Ethernet systems. Using Alternative A (not shown), power is carried over the so-called data pairs (1/2 and 3/6) using so-called “phantom feeding.” The so-called data pairs are used for data transmission in all, i.e., 10 MBit, 100 MBit, and Gigabit, Ethernet systems. PSE device 101 selects the connection method to employ, and the PoE standard requires that PD device 102 be able to use either connection method. Thus, PD device 102 has additional components, not shown, known to one of ordinary skill in the art, as are needed to comply with the requirements of the PoE standard, including, for example, another bridge circuit, similar to bridge circuit 111, connected to the data pairs.
Power supply 105 provides a 48V DC signal to PSE module 104. PSE module 104 contains the PSE control circuitry and provides a 48V DC differential signal to the spare pairs of cable 108, in particular, a 48V DC signal to wires 7 and 8 of cable 108, and a ground signal to wires 4 and 5 of cable 108. These polarities may also be reversed without departing from the PoE standard. If PSE module 104 were providing power using Alternative A (not shown), then PSE module 104 would provide a 48V DC differential signal to the data pairs of cable 108. PHY module 103 is powered by power supply 106, which provides 2.5V DC. PHY module 103 functions as the physical layer interface between signals provided to/from RJ45 interface 107 and signals provided via path 103a. Path 103a provides a connection via a 4-pin serial Gigabit medium-independent interface (SGMII) to and from a device at a network layer higher than the physical layer, such as an Ethernet media access controller (MAC) (not shown). RJ45 interface 107 uses center-tapped transformers to allow the transmission of power from PSE module 104 and/or data to/from PHY module 103, while simultaneously maintaining electrical isolation across RJ45 interface 107.
Cable 108 transmits electrical power and data from PSE device 101 to PD device 102, as well as data from PD device 102 to PSE device 101. RJ45 interface 113, like RJ45 interface 107, uses center-tapped transformers, such as transformer 114, to allow the transmission of power while simultaneously maintaining electrical isolation across RJ45 interface 113. The signals on the spare pairs have a data component if, for example, PoE system 100 uses Gigabit Ethernet. The data component of a signal on a wire pair is transmitted through RJ45 interface 113 to/from PHY module 110, which transforms the data for/from transmission via 4-pin SGMII path 110a. 
The DC signal component of the signal on the spare pairs goes to bridge circuit 111, which ensures that regardless of the polarity of the voltage on the spare pairs, the polarity of the voltage output by bridge circuit 111 is the same, i.e., that signal 111p provides the higher voltage (e.g., 48V) and signal 111m provides the lower voltage (e.g., ground). Signals 111p and 111m are provided to PD module 109 via PoE signature circuit 112. PoE signature circuit 112 contains circuitry used in performing PD signature functions such as detection and optional classification. If signature circuit 112, or its equivalent, is not present, and if PSE device 101 polls PD device 102, then no PD is detected, and PSE module 104 does not provide power. Optional classification indicates to PSE device 101 the expected power consumption of PD device 102 so that PSE module 104 can appropriately manage power requirements.
PD module 109 receives signals 111p and 111m and uses them to provide power to PHY module 110 as well as to other components of PD device 102 (not shown), while keeping PITY module 110 and the other components electrically isolated from DC signals 111p and 111m. PHY module 110 of PSE device 102 functions in substantially the same way as PITY module 103 of PSE device 101. In particular, PITY module 110 functions as the physical layer interface between signals provided to/from RJ45 interface 113 and signals provided via path 110a. Path 110a is a connection to an Ethernet MAC (not shown) via a 4-pin SGMII interface.